Effect of positive end-expiratory pressure and body position in unilateral lung injury

Positive end-expiratory pressure (PEEP), by increasing lung volume in acute lung injury, may recruit terminal air spaces in the involved regions, but may also distend noninvolved regions increasing extravascular lung water and worsening gas exchange. We investigated the effect of increasing levels of PEEP on arterial oxygenation in 26 anesthesized dogs with unilateral acid pneumonitis and studied the influences of gravity and distribution of the injury on this effect. Arterial PO₂ was consistently higher when the noninjured lung was dependent than in the supine or injured lung-dependent positions. Low levels of PEEP (5, 10 cmH₂O) improved arterial oxygenation and reduced intrapulmonary physiological shunt. However, 15 cmH₂O PEEP resulted in worsening of gas exchange, increased dead space ventilation, and diminished static compliance. The adverse effects of high levels of PEEP on arterial oxygenation were similar whether the injured lung was dependent or not and were evident at lower levels of PEEP in one group of dogs in which the unilateral injury was more diffuse and in which the upper and middle lobes were also involved. Thus, the compressive effects of high levels of PEEP on alveolar capillaries in the noninjured lung are influenced by the extent and distribution of injury in the injured lung, but not by local forces governing regional blood flow distribution.